1. Field of the Invention
This invention relates to a light source for reduced projection suitable as an exposure light source for transfer printing an extremely fine pattern on a semiconductor wafer.
2. Description of the Prior Art
The light source of this type is required to have a narrow line width of spectrum in order to provide a high resolution and it has been proposed to use an injection lock type excimer laser device as the light source.
As shown in FIG. 11, this laser device comprises an oscillator 10 acting as a stable resonator and an amplifier 20 acting as an stable resonator.
In the oscillator an oscillation occurs between mirrors 11 and 12 and the wavelength of a light beam is selected by a dispersion prism 13. Since the light beam is throttled by apertures 14 and 15 so that laser light having a narrow spectrum line width and a coherent beam characteristic can be produced. This laser beam is projected upon the amplifier through mirrors 17 and 18 with the result that the amplifier 20 undergoes a forced synchronous oscillation in a cavity mode.
The injection lock type excimer laser device can produce laser light having a narrow spectrum line width. However, since the transverse mode of the laser light is of a single mode, where the laser device is used as the light source for reduced projection, speckle (interference fringe) are formed thus failing to provide a high resolution.